CHIPP Plenary Meeting University of Geneva, June 12, 2008 W. Lustermann on behalf of the AX PET Collaboration INFN Bari, Ohio State University, CERN, University of Michigan, University of Oslo, INFN Roma, University of Valencia, PSI Villigen, ETH Zurich Introduction AX PET Concept Crystals Wave length shifting strips Photo detectors Mechanics Electronics Test setup
PET Positron Emission Tomography L Is recognized as the least invasive nuclear imaging technique It provides information about metabolic processes While other techniques like MRI or CT provide morphologic information How does it work A metabolic active molecule is marked with a radioactive isotope (β + emitter) Example: FDG fluorodesoxyglucose with 18 F The substance is enriched in metabolic active regions like cancer tissues Each emitted positron annihilates with an electron in the tissue, resulting in two back to back 511 kev γ s The photon pairs are detected in coincidence Using the positions of the photon pairs and the fact that they are back to back the intensity of the source is reconstructed Typical implementations are full body scanners, brain PET scanners (with important applications in neurology) and small animal PET scanners used in cancer research δ p α 2
Measured Parameters (Photon pair) Location: x 1, y 1, z 1 and x 2, y 2, z 2 Energy: E 1 and E 2 Time: t 1 and t 2 or Δt = t 1 t 2 Energy and time are required for photon pair selection and background reduction Pet with radial oriented crystals x,y resolution given by crystals cross section z resolution depends on crystal length L L Photon pair detection efficiency very important, in particular in clinical applications length of the crystals: L Attenuation length of the crystals: λ a ε 2 = 1 e L λ a 2 LYSO: λ a = 1.2 cm L = λ a ε 2 ~40% L = 2 * λ a ε 2 ~75% δ p α 3
Depth of interaction (DOI) δ p = L sin α is not measured Introduces a parallax error The resolution in the off center region degrades significantly L Solution: Measure DOI How: Change the geometry use long crystals oriented parallel to the scanner axe Use wave length shifting strips (WLS) to read the third coordinate δ p α 4
Details of scintillation and fluorescence light trapping X and Y coordinate are defined by the crystal dimensions Z coordinate is defined by the width of the WLS strip Light detection by novel photo detectors G APDs = MPPC x High PDE ~35% Very fast (~5ns peaking time) Immune to B field (MRI, CT) y Resolution in all three coordinates can be chosen, without compromising on the photon pair detections efficiency Detection of compton cascades is possible Increase efficiency and resolution CERN 13 March, 2008 W. Lustermann, ETH Zurich 5 z
Measurements were carried out and the results were published A. Braem et. Al, High Precision Axial Coordinate Readout for an Axial 3 D PET Detector Module using a Wave Length Shifter Strip Matrix, NIM A 580(2007), 1513 1521 A. Braem et. Al, Wave Length Shifter Strips and G APD Arrays for the Read Out of the z Coordinate in Axial PET Modules, NIM A 586 (2008), 300 308 WLS results WLS photoelectrons yield: ~80 for (511 kev photon absorbed) Axial coordinate resolution (digital): 2.8 mm (FWHM) using 3 mm wide WLS strips LYSO energy resolution 11.5% FWHM using G APDs and 511 kev equivalent X rays 6
AX PET Demonstrator Module 1 Module 2 Crystals WLS Crystals WLS 6 layers of 8 crystals 6 layers of 26 strips 6 layers of 8 crystals 6 layers of 26 strips 48 readout channels 156 readout channels 48 readout channels 156 readout channels Develop simulation and reconstruction software and perform measurements with the demonstrator The results from demonstrator will be used to validate a mathematical model of the scanner, which is developed in parallel. Based on the evaluated mathematical model we will be able to predict the expected performance obtainable in various scanner applications. 7
Crystal material: Manufacturer: Dimensions: All (116) crystals are delivered Chemical composition: Lu 9 YSiO 25 non hygroscopic Density: 7.1 g / cm 3 Absorption length: 1.2 cm Peak of emission spectrum: 420 nm Index of refraction at 420 nm: 1.81 Light yield: 32 photons / kev γ Decay time: 41 ns, single exponential 8
Results for 47 measured crystals Two PMT Burle 8850 (2 inch) Measure 511 kev γ s from a Na22 source Attenuation length of scintillation light λ = (41.9 ± 0.6) cm Counts LYSO Lambda Distribution 10 8 6 4 Lambda Entries 47 Mean 43.55 RMS 5.076 χ 2 / ndf 6.386 / 8 Constant 9.21 ± 1.86 Mean 41.93 ± 0.58 Sigma 3.071 ± 0.500 Number of p.e. Energy resolution (FWHM) Counts LYSO Nzero Distribution 12 10 8 6 4 N pe = 1287 ± 13 δ/e = (11.0 ± 0.4) % Nzero Entries 47 Mean 1288 RMS 73.48 χ 2 / ndf 1.194 / 4 Constant 12.03 ± 2.34 Mean 1287 ± 13.3 Sigma 77.78 ± 11.62 Counts 2 0 30 35 40 45 50 55 60 65 70 Lambda (cm) LYSO Resolution Distribution 14 Entries 47 Mean 4.695 12 RMS 0.134 10 χ 2 / ndf 2.336 / 3 Constant 11.42 ± 2.25 8 Mean 4.687 ± 0.030 Sigma 0.168 ± 0.031 6 4 R 2 2 0 1100 1200 1300 1400 1500 1600 Nzero 0 4 4.2 4.4 4.6 4.8 5 5.2 5.4 R (%) 9
transmission T WLS material: Manufacturer: Dimensions: 1.0 0.8 0.6 0.4 0.2 WLS sheet ELJEN EJ-280-10x LYSO, wavelength of max. emission 0.7 mm 1.1 mm 1.5 mm Shifts blue light into green Density: 1.023 g / cm 3 Absorption length: 1.2 cm Index of refraction: 1.58 Maximum of absorption: 425 nm Maximum of emission: 490 nm Decay time: 8.5 ns Quantum efficiency of fluorescent material: 0.86 % Doping: 10x with respect to standard 0.0 300 400 500 600 wavelength λ (nm) 10
Test setup Lecroy oscilloscope with GPIB readout Signal generator with 10ns pulse width Blue LED (420nm) and Light guide NIM module HV power supply PMT Hamamatsu R1450 Inject light at 2.5mm and up to 37.5mm with 5mm steps 61 samples tested Two points measured at 7.5mm and 32.5mm average loss : 6 p.e. over 25mm 7 32.5mm WLS 2 PMT WLS will be coated with Al at one end 7.5mm 11
Photo detectors: MPPC Manufacturer: Hamamatsu Hamatsu 3x3mm MPPC OCTAGON SMD Two different types for WLS and LYSO MPPC LYSO: S10362 33 50 C active area: 3 x 3 mm 2 3600 pixels of (50 μm) 2 Ceramic package 5.9 x 6.6 mm 2 MPPC WLS: custom made MPPC active area 3.22 x 1.19 mm 2 1200 pixels of (50 μm) 2 octagonal plastic package Operation voltage: 70 V Gain: 7.5 10 5 S10362 33 50 C 12
Two identical modules 6 layers of 8 LYSO bars 3.5 mm crystal pitch within a layer the layers are shifted by ½ the pitch with respect to each other 6 layers of 26 WLS strips 3.2 mm pitch within a layer LYSO crystals WLS strips MPPCs Carbon fiber support plates 13
MPPCs glued on LYSO crystals and on WLS strips Water cooled panels around MPPCs MPPCs pushed through a soft foam layer Patch panel PCBs for WLS and crystal readout Aluminum components are chromatized Kapton flex prints connect MPPCs to the patch panel PCBs 14
Fast amplifier VATA GP5 ASIC 64 (128) channels sparse mode 10 bit ADC Shaper 50 ns LYSO # 1 Output PAs GP5 Threshold 30 kev To GP5 LYSO # 2 Sum ( Or ) Sum ( Or ) Busy DAQ U th (VA) > U th (trig.) Disc. Trig. (10 ns) E th ~20 30 kev To GP5 Busy DAQ Disc. Trig. (10 ns) E th ~ 20 30 kev Output PAs Shaper 50 ns Busy DAQ OR Busy DAQ Disc. (100 ns) 450<E(keV)<550 Reset at end of the DAQ sequence AND Start busy DAQ OR AND No Yes Start Read Out Reset GP5 Readout VATA GP5 in sparse mode J. Seguinot E. Chesi 05/02/2008 15
LYSO crystal MPPC WLS strips Temperature sensors (AD590) Na 22 source LYSO crystals with PMT H8443 16
Spectrum of the Na 22 source measured with the MPPC in coincidence Peak position: 513. 2 kev FWHM: 61.5 kev Energy resolution: 12.0 % 17
Light in the WLS measured with the Na 22 source with the MPPC in coincidence Peak = 73 photo electrons in one WLS Light in the LYSO 18
Novel Concept of PET scanner axially oriented crystals with WLS strip readout of the z coordinate Separates photon pair detection efficiency and spatial resolution Principle is proven Detector components are selected Mechanics is being fabricated Electronics is under development First results with Na 22 source are very promising Start testing with the two detector modules this year 19